PhD defence Marc Pichel

The behaviour of magnetotactic bacteria in changing magnetic fields 

Marc Pichel is a joint degree PhD-Student in the research group Robotics and Mechatronics of the University of Twente. His supervisors are Professor Leon Abelmann (also Saarland University) and Professor Gijs Krijnen from the Faculty of Electrical Engineering, Mathematics and Computer Science. The co-operation for this joint degree is with the Saarland University in Germany.  

The observation of behaviour of magnetotactic bacteria (MTB) in changing magnetic fields can give significant direct and indirect information about their traits and biophysical properties. Both single and bulk experiment and analysis were performed in this study.

The single cel experiments were performed inside custom microfluidic chips designed to keep the MTB in focus, while a magnet field was applied using a permanent magnet mounted under a microscope stage. Observation and recording of the response allowed for off-line analysis of the trajectories. This analysis has shown that the cells respond differently to varying magnitudes of magnetic field strength.

Furthermore, from simulations and experiments we have found that the drag of the MTB had been underestimated, which lead to additional macroscopic experiments relating morphological traits to more rotational drag profiles. These experiments were done in a vat of silicone oil using 3D-printed models of varying spheroid- and spirillum-based morphologies. The models were based on scanning electron microscope images of actual MTB. Analysis of these experiments elucidated the contribution of traits not included in existing models for rotational drag.

The bulk analysis was performed in a custom made optical density meter, specifically designed to combine magnetic field orientations with photo spectrometry. From our observation we could derive the magnetic response, both absolute and relative, of a given culture or sample of MTB. Additionally, the response time of a given batch could also be measured, relating the magnetic dipole moment with the rotational drag. This allowed distinguishing between different quality and quantity of cultures, as well as long term and continuous observation of a culture in growth.

In spite of having found new traits by which one can more accurately calculate the rotational drag profile, the length of an object still remains the dominate factor when balancing magnetic torque and drag force. Our model does allow for predicting more accurately the rotational drag of objects with shapes similar to MTB in Stokes flow or under low Reynolds number conditions.